Re: [PATCH v4 0/2] mm: slub: Enhanced debugging in slub error

Vlastimil Babka
2025-02-27T16:26:32Z

On 2/27/25 17:12, Vlastimil Babka wrote:
> On 2/26/25 09:11, Hyesoo Yu wrote:
>> Dear Maintainer,
>>
>> The purpose is to improve the debugging capabilities of the slub allocator
>> when a error occurs. The following improvements have been made:
>>
>> - Added WARN() calls at specific locations (slab_err, object_err) to detect
>> errors effectively and to generate a crash dump if panic_on_warn is enabled.
>>
>> - Additionally, the error printing location in check_object has been adjusted to
>> display the broken data before the restoration process. This improvement
>> allows for a better understanding of how the data was corrupted.
>>
>> This series combines two patches that were discussed seperately in the links below.
>> https://lore.kernel.org/linux-mm/20250120082908.4162780-1-hyesoo.yu@samsung.com/
>> https://lore.kernel.org/linux-mm/20250120083023.4162932-1-hyesoo.yu@samsung.com/
>>
>> Thanks you.
>
> Thanks. On top of things already mentioned, I added some kunit suppressions
> in patch 2. Please check the result:
>
> https://web.git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-6.15/fixes-cleanups

What do you think about the following patch on top?

---8<---
From c38dadde6293cacdb91f95afc3615c22dec5830a Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Thu, 27 Feb 2025 16:05:46 +0100
Subject: [PATCH] mm, slab: cleanup slab_bug() parameters

slab_err() has variadic printf arguments but instead of passing them to
slab_bug() it does vsnprintf() to a buffer and passes %s, buf.

To allow passing them directly, turn slab_bug() to __slab_bug() with a
va_list parameter, and slab_bug() a wrapper with fmt, ... parameters.
Then slab_err() can call __slab_bug() without the intermediate buffer.

Also constify fmt everywhere, which also simplifies object_err()'s
call to slab_bug().

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
mm/slub.c | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index a9a02b4ae4d6..d94af020b305 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1017,12 +1017,12 @@ void skip_orig_size_check(struct kmem_cache *s, const void *object)
set_orig_size(s, (void *)object, s->object_size);
}

-static void slab_bug(struct kmem_cache *s, char *fmt, ...)
+static void __slab_bug(struct kmem_cache *s, const char *fmt, va_list argsp)
{
struct va_format vaf;
va_list args;

- va_start(args, fmt);
+ va_copy(args, argsp);
vaf.fmt = fmt;
vaf.va = &args;
pr_err("=============================================================================\n");
@@ -1031,8 +1031,17 @@ static void slab_bug(struct kmem_cache *s, char *fmt, ...)
va_end(args);
}

+static void slab_bug(struct kmem_cache *s, const char *fmt, ...)
+{
+ va_list args;
+
+ va_start(args, fmt);
+ __slab_bug(s, fmt, args);
+ va_end(args);
+}
+
__printf(2, 3)
-static void slab_fix(struct kmem_cache *s, char *fmt, ...)
+static void slab_fix(struct kmem_cache *s, const char *fmt, ...)
{
struct va_format vaf;
va_list args;
@@ -1088,12 +1097,12 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
}

static void object_err(struct kmem_cache *s, struct slab *slab,
- u8 *object, char *reason)
+ u8 *object, const char *reason)
{
if (slab_add_kunit_errors())
return;

- slab_bug(s, "%s", reason);
+ slab_bug(s, reason);
print_trailer(s, slab, object);
add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);

@@ -1129,15 +1138,14 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
const char *fmt, ...)
{
va_list args;
- char buf[100];

if (slab_add_kunit_errors())
return;

va_start(args, fmt);
- vsnprintf(buf, sizeof(buf), fmt, args);
+ __slab_bug(s, fmt, args);
va_end(args);
- slab_bug(s, "%s", buf);
+
__slab_err(slab);
}

@@ -1175,7 +1183,7 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
s->inuse - poison_size);
}

-static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
+static void restore_bytes(struct kmem_cache *s, const char *message, u8 data,
void *from, void *to)
{
slab_fix(s, "Restoring %s 0x%p-0x%p=0x%x", message, from, to - 1, data);
@@ -1190,7 +1198,7 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,

static pad_check_attributes int
check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
- u8 *object, char *what, u8 *start, unsigned int value,
+ u8 *object, const char *what, u8 *start, unsigned int value,
unsigned int bytes, bool slab_obj_print)
{
u8 *fault;
--
2.48.1